The most critical phase of SEPTA's $10.4 million track replacement project under way between Penn Center Suburban Station and points a mile south and a mile west of 30th Street Station will affect rail traffic through Lansdale and Doylestown.

The work will start tomorrow evening, and last through August, a SEPTA spokesman said yesterday.

Since last September, SEPTA has been replacing ties on either side of 30th Street Station, but now the work will be at the station itself. The six tracks that go through 30th Street will be taken out of service one at a time, and each for 10 days.

The work beginning tomorrow will cause delays to trains headed through 30th Street toward center city on SEPTA Regional Rail Routes R5, R7 and R8. Through the summer, each track taken out of service will have its own particular service impact and SEPTA said it will do everything it can to keep riders updated through printed material, public address announcements and the media.

The 30th Street construction involves total replacement of a track system built in the early 1930s. Tracks and roadbed will be removed down to the roof of the 30th Street Station building. After the roof is waterproofed, new roadbed and tracks will be installed. During each track shutdown there will be significant changes in train service and possible delays.

For the first 10 days, from 8 p.m. tomorrow, until 4:45 a.m. May 24, Track One on Platform A will be closed causing most Route R5 trains coming in from Paoli to be rerouted onto Track Two. This will greatly increase traffic on Track Two since inbound Route R7 and R8 trains will continue to use that track. SEPTA advised commuters that delays of up to 10 minutes can be expected by riders of all three routes.

In addition, SEPTA said, inbound delays at 30th Street will affect outbound service from Penn Center and Market East for Route R5, R7 and R8 riders headed toward Lansdale-Doylestown, Chestnut Hill East and Fox Chase.

SEPTA also advised riders boarding trains at 30th Street toward center city to listen very carefully for public address announcements, especially during the weekday evening rush hours. The trains on Routes R5, R6, R7 and R8 that usually start their trips at 30th Street from Track One or Track Two might be rerouted to Track Three on Platform C or Track Six on Platform B.

The first objective of this project, SEPTA said, is to preserve train service on the western side of Philadelphia. According to SEPTA, the heavily used railroad infrastructure now being replaced after 55 years of service has been estimated to deteriorate to an unsafe and, therefore, unusable condition within the next year or two.